High Temperature Oxidation and Electrochemical Studies Related to Hot Corrosion

摘要

This research has examined the transport mechanisms relevant to hot corrosion.Three different approaches were used to elucidate this problem: an examination of the corrosion of the base metal, an exploration of the transport properties in Na2SO4, and an investigation in the role of defects in the transport behavior of a protective coating. The transport properties of molten Na2SO4 have been investigated to aid further understanding of hot corrosion processes at 1 1 73 K. Tests were conducted to determine: the effect of height of Na2SO4 melts, the effect of electronic short circuiting the Na2SO4 melts with gold wires, and the effect of presaturating the salt melt with corrosion products in the hot corrosion process. Basically, these tests showed that neither ionic nor electronic transport in the bulk melt controls the hot corrosion process. However, the amount of Na2SO4 present, which is directly proportional to the height of the melt, does affect the rate of corrosion. This was demonstrated by the decrease in rate observed when the melt was presaturated with the corrosion products. An A.C. impedance technique for the total electrical conductivity, the potentiostatic polarization method for ionic transport numbers, and the steady state polarization method of Wagner and Hebb for the electronic conductivity were employed as a function of Na2O activity in the melt by controlling the gas atmosphere. These results showed that the conductivities of the pure Na2SO4 were of the order of 0.232 (ohm-cm)-l and varied only slightly with Na2SO4, activity changes. From the polarization measurements, on the pure Na2SO4, it was found that electron conductivity was much higher than electron hole conductivity. The transport numbers were = 7.Oxl0-3 and = 5.7x